Multiple flash messages per type support

Basic support for multiple flash messages per type, as per discussion
https://github.com/yiisoft/yii2/pull/1250#issuecomment-52454763

Tested in advanced and basic app use cases.

It turned out that advanced template alert widget already works fine with array of messages.

The approach used in basic app:
```
 Yii::$app->session->setFlash('contactFormSubmitted');
```
is also not broken.


The remove-after-access and remove-on-the-next page logic should work fine but I did not test it thoroughly.
What if it's not empty and is string? `$_SESSION[$key][] = $value;` won't work.